Ti possesses a strong avidity to elements O and N. These three elements are easy to combine during steel-making process so as to form minute inclusions, such as TiOx and TiN etc. These minute inclusions will reduce purity of the steel, resulting in clogging at a continuous casting nozzle or floater in mold, which in turn affects all sorts of performance of the steel. Therefore, a strict standard is demanded on the content of Ti in a part of special steel grades, e.g., electrical steel, bearing steel, spring steel etc., which want no more than 30 ppm, even 15 ppm.
The main traditional methods for reducing content of Ti in steel are as follows.
By strictly controlling content of Ti in raw and auxiliary materials, such as hot metal, ferroalloy, tundish cover etc and top ladle slag, and by means of converter-twined and secondary slag formation etc., dragin of the content of external Ti in steel can be reduced, thereby avoiding increase of the content of Ti after liquid steel deoxidation and alloying. Such a method is disclosed in for example, Japanese Patent Application Laid-open No. 7-173519, No. 2002-322508, No. 2002-105578 and No. 2004-307942. But such a method has a strict demand on quality of the above-described raw and auxiliary materials, which significantly increases manufacturing cost and smelting period of the steel, and in order to prevent TiO2 in top ladle slag from being deoxidized, it is necessary to control the content of aluminum in steel, which is commonly controlled to no more than 0.1%.
Also, by means of modification of the top ladle slag in smelting and refining process, as well as adjustment on basicity of the slag and oxidability of liquid steel, and the like, distribution ratio of Ti between the slag steel can be improved, which is disclosed in for example, Japanese Patent Application Laid-open No. 5-86413 and No. 2003-73726 etc. The disadvantage of this method is that the liquid steel is not desulfurized, and the content of Al in steel is relatively low, which is commonly controlled to no more than 0.1%.